booster

Also known as a booster rocket. (1) A stage
of a launch system used to provide an initial thrust
greater than the total liftoff weight. (2) A rocket engine, either solid-
or liquid-fueled, which assists the normal propulsive system or sustainer
engine of a rocket or aeronautical vehicle in some phase of its flight.
(3) A rocket used to set a launch vehicle in motion before another engine
takes over. A booster assembly is a structure that supports
one or more booster rockets.